Laser beams are coherent light beams with a single wavelength. This characteristic makes them ideal light sources to transmit bundled energy or to carry optical signals. Both applications are of growing importance in information technology, medicine, physics and other fields of technology and sciences. The wavelength of the laser light can vary from infra red to ultra violet.
Several methods to generate laser beams have been developed to produce laser light of distinct wavelengths. Lasers made from gaseous and solid-state materials can directly produce beams with relatively wide bandwidths of wavelengths. To produce beams that fulfill special demands for tunability or frequency stability techniques such as nonlinear frequency conversion are mainly used.
A nonlinear frequency converter is capable of converting some fraction of the light from a beam or beams entering the converter into a beam or beams of different wavelengths, which then radiate from the converter. This process is achieved in a several forms. Three of them are: firstly, in a second harmonic generation process a first beam with distinct wavelength enters the converter and a second beam with a different wavelength is exiting; secondly, in a sum or difference frequency mixing process two beams of different wavelength enter and one beam with a wavelength representing the sum or the difference of the entering beams is exiting; thirdly, in an optical parametric generation or optical parametric oscillation process one beam is entering and two different wavelength beams exiting.
The converter is typically made of crystalline material or other materials with large second-order nonlinear susceptibilities. The converter may include mirrors at the front and back end where beams enter and radiate. The mirrrors form an optical cavity and provide optical feedback. The entering beam or beams produce an oscillating polarization in the converter. As a result, a beam with a new and different wavelength or wavelengths radiates from the converter. Fractions of the entering beam or beams that are not converted to a different wavelength typically exit the converter along with the converted beams. It is desirable to reduce the amount of radiating unconverted beams to increase the efficiency of the wavelength converter.
The wavelength conversion process is distributed onto a number of regions arrayed over the entire length of the converter. To achieve high conversion efficiency the contributions from every region has to add up in phase, which is called "phasematching"
In an efficient converter the phasematching between the nonlinear polarization and the radiating beam frequencies is at a maximum.
A technique to achieve phasematching is called "quasiphasematching". Thereby, the regions of the converter are diverted in structure groups that have alternating periodically or aperiodically modulated nonlinear susceptibilities. This structure groups are called domains.
For zincblende semiconductor materials such as Gallium Arsenide and Zinc Selenide, the nonlinear susceptibility of the waveguide can be modulated by a periodically alternating change in the crystallographic orientation of the domains. More specifically, inversion of a zincblende crystalline structure changes the sign of the nonlinear susceptibility. In zincblende semiconductors a grating along the direction of the propagating laser is introduced to the structure groups.
By introducing an additional grating along the direction of propagating laser beam to the domains the quasiphasematched frequency conversion is additionally enhanced. quasiphasematched frequency conversion can be obtained.
The positive and negative domain make up one region which is the fundamental unit of repetition. Each region has a length set by the refractive index of the semiconductor material. The refractive index varies with the wavelength of the propagating laser beam.
The duty cycle of the positive and negative domain within this unit is ideally 50/50 to achieve highest efficiency. In some cases more complicated nonperiodic grating structures can be implemented to perform specialized functions, such as chirped gratings for temporal pulse compression or multiple gratings for simultaneous frequency conversion to multiple different wavelengths.
The periodic inverted crystallographic orientation that enables quasiphasematching of nonlinear frequency conversion can be used with two device implementations, so called "waveguide" and "bulk" devices.
A typical Waveguide device consists of a stack of layers of varying refractive index, which serve to confine the laser radiation as it propagates through the device. These devices are useful in applications such as telecommunications, where low power and high efficiency is required. The total thickness of all the layers in such a waveguide device is smaller or equal 10 .mu.m.
A Bulk device utilizes much thicker apertures, extending from a few 100 .mu.m up to the cm range in size. In this bulk implementation the laser beams are simply focused with external optical lenses through the converter and no refractive index variations are utilized to enhance guiding of the laser beams. The bulk devices are useful for handling high power laser beams.
To keep the scattering of the beams within the waveguide low, the boundaries between the crystallographyically inverted structures have to be of good optical quality.
U.S. Pat. No. 5,434,700 describes an optical wavelength converter, whereby a number of regions with different non-linear optical susceptibilities are arranged to form an optical waveguide. The patent discloses a bonding method to fabricate the two groups of crystallographically inverted domains. A first GaAs layer deposited on a first wafer has a top layer of indium gallium phosphate (InGaP). The first wafer is flipped upside down and bonded with the InGaP layer on a second GaAs layer of a second wafer. First and second GaAs layer are grown with the same crystallographic orientation. Flipping the first wafer upside down inverts the crystallographic orientation of the first GaAs layer relative to the second GaAs layer. The bonding process is performed by atomic rearrangement, as described in U.S. Pat. No. 5,207,864. In a consecutive step, additional layers of the first wafer only required for the bonding process are removed to leave a 10-nm layer of inverted GaAs bound with a 20-nm layer of InGaP on the second GaAs layer. A mask is patterned over the inverted first GaAs layer leaving apertures and masked areas. In selective etching steps the top GaAs layer and the InGaP layer are removed leaving a pattern of inverted GaAs domains on top of the exposed second GaAs layer. Epitaxially grown layers of GaAs based substrates take on the crystallographic orientation of both layers producing regions of alternating crystallographic orientation.
Another method to create alternating crystallographic domains is described in US. Pat. No. 5,434,700. Masked ion implantation is used to selectively destroy crystalline asymmetry. As a result, a zinc blend based layer is converted in domains with differing optical properties to provide the regions for the wavelength conversion. The method does not invert the crystallographic orientation which is defined during the growth process.
U.S. Pat. No. 5,734,494 also describes arrays of periodic crystallographically inverted GaAs as part of the invention. No fabrication method is disclosed.
Arrays of periodic crystallographically inverted zincblende semiconductors are the central design material of wavelength converters.
However, reliable and simple methods to fabricate wavelength converters from GaAs or zincblende semiconductors materials have not been identified. The current invention addresses this need.